Previous research has linked fright and anxiety to poor heart health, but this is the first to make a connection between depression and the heart, said lead researcher Dr. Parveen Garg. He's an assistant professor of clinical medicine with the University of Southern California's Keck School of Medicine in Los Angeles.

"Our research emphasizes the link between mental health and cardiovascular health," Garg said. "Our mental health and our heart health are very intertwined."

At least 2.7 million Americans live with atrial fibrillation, the most common heart rhythm disorder, according to the American Heart Association (AHA).

Also known as a-fib, the condition involves a quivering or irregular heartbeat that allows blood to pool and clot in the upper chambers of the heart, which increases stroke risk.

Untreated atrial fibrillation doubles the risk of heart-related deaths and is associated with a five-fold increased risk for stroke, the AHA says.

It's well understood that stress-inducing emotional states such as fright and anxiety affect heart health, possibly by triggering the body's "fight-or-flight" response, said Dr. Russell Luepker. He's a professor with the University of Minnesota School of Public Health and was not connected with the study.

A surge of hormones are released during that response, causing short-term alterations to heart rhythm that could have long-term consequences over time, the researchers said.

Depression is another state of emotional distress that has been linked to increases in stress hormones and inflammation. But it has not been linked to heart health because it is more insidious and less obviously stressful than an anxiety attack or fit of rage, Garg explained.

To see whether depression hurts heart health, Garg and his colleagues analyzed data on more than 6,600 participants in a long-term, multi-ethnic study of heart health. This study assessed symptoms of depression when participants entered the trial, and also asked whether they were taking antidepressants.

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The researchers found an increased risk of developing atrial fibrillation during a decade-long follow-up period if the participants reported signs of depression, compared with those having no depression, Garg said.

The increased risk held even after researchers controlled for other known risk factors for a-fib, including smoking, obesity and high blood pressure.

Luepker, an AHA spokesman, noted that the increased a-fib risk associated with depression was not "enormous."

But the study raises enough concern that doctors who want to protect their patients' heart health should keep an eye on their chronic emotional states, Luepker said.

"You need to keep an eye on your depressed patients, because it's conceivable they could be at somewhat higher risk for atrial fibrillation," Luepker said.

It should be noted that the study only found an association between depression and increased risk of atrial fibrillation. It did not prove cause and effect.

Garg was to present the findings Thursday at an AHA meeting, in New Orleans. Research presented at meetings is considered preliminary until published in a peer-reviewed journal.